Objectives: (1) To elucidate the mechanisms governing the regulation of hepatic hematopoiesis during mammalian development, (2) to investigate the mechanisms of alterations in the regulation of delta- aminolevulinic acid (ALA) synthetase during hepatocarcinogenesis induced by exogenous agents, (3) to characterize and study the effects of environmental agents on purified preparations of ALA synthetase from adult, fetal and neoplastic rat liver, (4) to determine the correlation between biochemical abnormalities associated with porphyrin overproduction and the clinical symptoms of porphyria, and (5) to provide a pharmacological and biochemical basis for predicting human responses to environmental agents which interfere with hepatic heme biosynthesis.